Phase modulating systems are known from the state of the art. In a phase modulating system, a signal generator providing the signal that is to be modulated is connected to a phase shifter. The signal generator can be an analog oscillator providing a sine wave or digital oscillator providing a square wave. The phase shifter applies a phase shift to a signal received from the signal generator and provides the phase modulated signal as output signal.
The phase shifter can be controlled by a control signal indicating the amount of the respectively desired phase shift. The control signal can be either analog or digital.
In a phase modulating system employing an analog control, an analog phase shifter is controlled by an analog control signal, for instance a control voltage. It is difficult, however, to implement a fully analog phase shifter that fulfills stringent precision requirements for modulation. In particular, the phase shift applied by an analog phase shifter will usually not follow its control voltage linearly.
In a phase modulating system employing a digital control, one of several available carriers having different phases is selected by a digital control signal. In order to provide several carriers with different phases, the phase shifter can be realized for example by a delay line dividing the generator signal into several carriers with the desired phases.
For illustration, a schematic block diagram of a phase modulating system controlled by an n-bit control signal is presented in FIG. 1. In this system, an oscillator 10 is connected via a delay line 11 and a multiplexer 12 to the output terminal ‘Out’ of the system. A single line connecting the delay line 11 and the multiplexer 12 represents a bus of 2n parallel lines. Further, an input terminal ‘In’ is connected to a control input of the multiplexer 12. The oscillator 10 provides to the delay line 11 a radio frequency signal which is to be modulated. The delay line 11 delays the received signal and provides 2n output signals to the multiplexer 12, each output signal being delayed by a different value. The multiplexer 12 constitutes a switching component which forwards the delayed signal corresponding to the value of an n-bit control signal received via the input terminal ‘In’ to the output terminal ‘Out’.
A possible structure of the delay line 11 is shown in more detail in the block diagram of a phase modulating system presented in FIG. 2.
The system of FIG. 2 comprises again an oscillator 20. The output of the oscillator 20 is connected to the input of a first delay element 21, the output of the first delay element 21 is connected to the input of a second delay element 22, and the output of the second delay element 21 is connected to a third delay element 23. The delay elements 21, 22, 23 form a 2-bit delay line.
When the oscillator 20 provides a signal to the delay line, the signal passes through the delay elements 21, 22, 23 and is delayed by each of the delay elements 21, 22, 23 by the same amount. The output of the oscillator 20 constitutes a first selectable output Out0, the output of the first delay element 21 constitutes a second selectable output Out1, the output of the second delay element 22 constitutes a third selectable output Out2, and the output of the third delay element 23 constitutes a fourth selectable output Out3. One of these outputs Out0 to Out3 can be selected by a switching component (not shown) according to the value of a 2-bit control signal. To this end, each possible value of the 2-bit control signal is associated to another one of the outputs Out0 to Out3.
In general, the total delay applied by a delay line to a signal received by a signal generator should be sufficiently large to cover one period of the generator signal. On the other hand, the number of bits required in the modulating control signals determines the number of delay elements in the delay line. That is, if the number of bits is n, the number of delay elements should be 2n−1. In FIG. 2, the number of bits of the control signal is n=2, thus three delay elements 21, 22, 23 are needed.
If the frequency of the oscillator signal is for instance 2 GHz and the number of bits of the control signal is 8, the required delay is 500 ps/256=1.95 ps. This is a very short time even for a simple inverter and, hence, requires a very fast processing. Obviously, the situation gets even more critical when the number of bits of the control signal is 9 or 10. It is difficult to implement the large number of small delays which is required for high frequency generator signals and control signals having a high numbers of bits.
An additional problem relating to the required number of small delays results from the fact that the required delay for each delay element depends on the frequency of the signal provided by the signal generator. Thus, if the frequency changes, the delay values of the delay elements should also change, lest the signal deteriorates. If the signal quality at the nominal frequency is very good, it may be possible to tune the frequency over a narrow band and still preserve the signal quality within the required specifications, but a tuning over a large band is not possible with acceptable results.
The allowed tuning range could be increased by using several delay lines in parallel, each of which would have a different center frequency. The respective active delay line would then be the one of which the nominal frequency is closest to the desired carrier frequency. This approach has the disadvantage, however, that it requires even more small delay elements. Another way to solve the problem is to use tunable delay elements. The tuning range of the tunable delay elements might not be wide enough, though. Moreover, as the control of tunable delay elements is analog, it may suffer from non-linearity.
Some conventional modulators, such as an IQ (In-phase/Quadrature) modulator, could equally be used to accomplish a phase modulation. However, conventional modulators are more complex than systems which are based on phase shifters and may consume more power. They may also generate more noise than a phase shifter.